Ecological Effects

Ecological risk assessment is patterned after human health risk assessment but is more complex. As a first step in the analysis, the ecological stressors are identified; then the ecosystem potentially impacted is determined. Ecological stressors can be categorized as chemical (e.g., toxic chemicals released into the atmosphere), physical (e.g., habitat destruction through logging), or biological (e.g., the introduction of an exotic species).

The Ecology and Welfare Subcommittee of the U.S. EPA Science Advisory Board has developed a method for ranking ecological problems (Science Advisory Board 1990). The subcommittee's approach is based on a matrix of ecological stressors and ecosystem types (Harwell and Kelly 1986). Risks are classified according to the following:

• Type of ecological response

• Intensity of the potential effect

• Time scale for recovery following stress removal

• Spatial scale (local or regional biosphere)

• Transport media (air, water, or terrestrial)

The recovery rate of an ecosystem to a stressor is a critical part of risk assessment. In an extreme case, an ecological stress leads to permanent changes in the community structure or species extinction. The subcommittee classifies ecosystem responses to stressors by changes in the following:

Biotic community structure (alteration in the food chain and species diversity) Ecosystem function (changes in the rate of production and nutrient cycling) Species population of aesthetic or economic value Potential for the ecosystem to act as a route of exposure to humans (bioaccumulation)

Determining potential risks and their likely effects is the first step in ecological assessment. Many stressors can be cumulative, finally resulting in large-scale problems. Both habitat degradation and atmospheric change are examples of ecological impact that gain attention.

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